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Extractive Distillation

Until recently, energy and mass-transfer processes in an actual distillation column were considered too complicated to be readily modeled in any direct way. This difficulty was circumvented by the equilibrium-stage model, developed by Sorel in 1893, in which vapor and liquid streams leaving an equilibrium stage are in complete equilibrium with each other andthermodynamic relations can be used to determine the temperature of and relate the concentrations in the equilibrium streams at a given...

Although much progress has been made in identifying the chemical species present in petroleum, it is generally sufficient for purposes of design and analysis of plant operation of distillation to characterize petroleum and petroleum fractions by gravity, laboratory-distillation curves, component analysis of light ends, and hydrocarbon-type analysis of middle and heavy ends. From such data, as discussed in the Technical Data Book Petroleum Refining American Petroleum Institute (API), Washington...

Consider the closed-loop response during the dynamic distillation of an ideal binary mixture in the column shown in Fig. 13-107, under two assumptions of constant relative volatility at a value of 2.0 and constant molar vapor flow for a saturated liquid feed to tray Ns. Following the development by Luyben (op. cit.), it is not necessary to include energy-balance equations for each tray or to treat temperature and pressure as variables. Overhead vapor leaving top tray NT is totally condensed for...

Typical equipment configurations for the distillation of crude oil and other complex hydrocarbon mixtures in a crude unit, a catalytic-cracking unit, and a delayed-coking unit of a petroleum refinery are shown in Figs. 13-87, 13-88, and 13-89. The initial separation of crude oil into fractions is conducted in two main columns, shown in Fig. 13-87. In the first column, called the atmospheric tower or topping still, partially vaporized crude oil, from which water, sediment, and salt have been...

Although the SC and inside-out methods are reasonably robust, they are not guaranteed to converge and sometimes fail, particularly for very nonideal liquid solutions and when initial guesses are poor. A much more robust, but more time-consuming, method is differential arclength homotopy continuation, the basic principles and applications of which are discussed by Wayburn and Seader Comp. Chem. Engng., 11, 7-25 1987 Proceedings Second Intern. Conf. Foundations of Computer-Aided Process Design,...

Flow, calculations are readily made of remaining values of Vj and Lj, from which initial values of vi,j and j are obtained from Eqs. 13-89 and 13-90 after obtaining approximations of xij and ,j from steps 4, 5, 8, 9, and 10 of the SR method. Alternatively, a much cruder but often sufficient estimate of Xy and yij is obtained by flashing the combined column feeds at average column pressure and a vapor-to-liquid ratio that approximates the ratio of overhead plus vapor-sidestream flows to bottoms...

Rigorous Computer-Based Calculation Procedures It is obvious that a set of curves such as shown in Fig. 13-104 for a binary mixture is quite tedious to obtain by hand methods. The curves shown in Fig. 13-99 for a multicomponent batch distillation are extremely difficult to develop by hand methods. Therefore, since the early 1960s, when large digital computers became available, interest has been generated in developing rigorous calculation procedures for binary and multicomponent batch...

FIG. 13-84 Comparison of ASTM, TBP, and EFV distillation curves for kero- batch distillation equipment that can achieve a good degree of fractionation is usually considered suitable. In general, TBP distillations are conducted in columns with 15 to 100 theoretical stages at reflux ratios of 5 or greater. Thus, the new ASTM D 2892 test method, which involves a column with from 14 to 17 theoretical stages and a reflux ratio of 5, essentially meets the minimum requirements. Distillate may be...

The choice of the appropriate azeotropic distillation method and the resulting flowsheet for the separation of a particular mixture are strong functions of the separation objective. For example, it may be desirable to recover all constituents of the original feed mixture as pure components, or only some as pure components and some as azeotropic mixtures suitable for recycle. Not every objective may be obtainable by azeotropic distillation for a given mixture and portfolio of candidate...

The mathematical models presented earlier for rigorous calculations of multistage, multicomponent distillation-type separations assume that equilibrium with respect to both heat and mass transfer is attained at each stage. Unless temperature changes significantly from stage to stage, the assumption that vapor and liquid phases exiting from a stage are at the same temperature is generally valid. However, in most cases, equilibrium with respect to mass transfer is not a valid assumption. If all...

Two general procedures are available for designing fractionators that process petroleum, synthetic crude oils, and complex mixtures. The first, which was originally developed for crude units by Packie Trans. Am. Inst. Chem. Eng. J., 37, 51 1941 , extended to main fractionators by Houghland, Lemieux, and Schreiner Proc. API, sec. III, Refining, 385 1954 , and further elaborated and described in great detail by Watkins op. cit. , utilizes material and energy balances, with empirical correlations...

For a binary mixture, pressure and temperature fix the equilibrium vapor and liquid compositions. Thus, experimental data are frequently presented in the form of tables of vapor mole fraction y and liquid mole fraction x for one constituent over a range of temperature T for a fixed pressure P or over a range of pressure for a fixed temperature. A compilation of such data, mainly at a pressure of 101.3 kPa 1 atm, 1.013 bar , for binary systems mainly nonideal is given in Table 13-1. More...

FIG. 13-75 Number of theoretical stages versus solvent-to-feed ratio for extractive distillation. a Close-boiling vinyl acetate-ethyl acetate system with phenol solvent. b Azeotropicacetone-methanol system with water solvent. no longer be achieved and the distillate purity actually decreases for a given number of stages LaRoche et al., AIChE J., 38, 1309 1992 . The difference between Rmin and Rmax increases as the S F ratio increases. Large amounts of reflux lowers the solvent concentration in...

Ketones and alcohols Table 13-21 , solvents favored for the recovery of methanol in the bottoms would come from groups more polar than methanol, such as acids, water, and polyols. Turning to the Robbins Chart Table 13-15 , favorable groups are amines, alcohols, polyols, and water since these show expected positive deviations for acetone and zero or negative deviations for methanol. For reversing the natural volatility, solvents should be chosen that are less polar than acetone, such as ethers,...

Introduction Reactive distillation is a unit operation in which chemical reaction and distillative separation are carried out simultaneously within a fractional distillation apparatus. Reactive distillation may be advantageous for liquid-phase reaction systems when the reaction must be carried out with a large excess of one or more of the reac-tants, when a reaction can be driven to completion by removal of one or more of the products as they are formed, or when the product recovery or...

FIG. 13-65 Valeric acid-water separation with formic acid. a Mass balances on distillation region diagram. b Conceptual sequence. be crossed. This may be done by mixing some external stream with the original feed stream in one region such that the resulting composition is in another region for further processing. However, the external stream must be completely regenerated, and mass-balance observed. For example, it is not possible to break a homogeneous binary azeotrope simply by adding one of...

In distillation operations, separation results from differences in vapor-and liquid-phase compositions arising from the partial vaporization of a liquid mixture or the partial condensation of a vapor mixture. The vapor phase becomes enriched in the more volatile components while the liquid phase is depleted of those same components. In many situations, however, the change in composition between the vapor and liquid phases in equilibrium becomes small so-called pinched condition , and a large...

Introduction Extractive distillation is a partial vaporization process in the presence of a miscible, high-boiling, non-volatile massseparation agent, normally called the solvent, which is added to an azeotropic or nonazeotropic feed mixture to alter the volatilities of the key components without the formation of any additional azeotropes. Extractive distillation is used throughout the petrochemical- and chemical-processing industries for the separation of close-boiling, pinched, or azeotropic...

At low-to-moderate pressure ranges typical of most industrial applications, the fundamental composition relationship between the vapor and liquid phases in equilibrium can be expressed as a function of the total system pressure, the vapor pressure of each pure component, and the liquid-phase activity coefficient of each component i in the mixture In systems that exhibit ideal liquid-phase behavior, the activity coefficients, y , are equal to unity and Eq. 13-124 simplifies to Raoult's law. For...

Separation operations achieve their objective by the creation of two or more coexisting zones which differ in temperature, pressure, composition, and or phase state. Each molecular species in the mixture to be separated reacts in a unique way to differing environments offered by these zones. Consequently, as the system moves toward equilibrium, each species establishes a different concentration in each zone, and this results in a separation between the species. The separation operation called...

FIG. 13-14 K values K y x in light-hydrocarbon systems. a Low-temperature range. DePriester, Chem. Eng. Prog. Symp. Sec. 7, 49,1 1953 . Preferred analytical correlations are less empirical in nature and most often are theoretically based on one of two exact thermodynamic formulations, as derived in Sec. 4. When a single pressure-volume-temperature PVT equation of state is applicable to both vapor and liquid phases, the formulation used is where the mixture fugacity coefficients lt t gt L for...

The widespread availability and utilization of digital computers for distillation calculations have given impetus to the development of analytical expressions for K values. McWilliams Chem. Eng., 80 25 , 138 1973 presents a regression equation and accompanying regression coefficients that represent the DePriester charts of Fig. 13-14. Regression equations and coefficients for various versions of the GPA convergence-pressure charts are available from the GPA. FIG. 13-13 Vapor-liquid equilibrium...

The simplest form of distillation involves boiling a multicomponent liquid mixture batchwise in a single-stage still pot. At any instant in time the vapor being generated and removed from the pot is assumed to be in equilibrium with the remaining liquid assumed to be perfectly mixed in the still. Because the vapor is richer in the more volatile components than the liquid, the composition and temperature of the liquid remaining in the still changes continuously over time and moves progressively...

Although the widely used equilibrium-stage models for distillation, described above, have proved to be quite adequate for binary and close-boiling, ideal and near-ideal multicomponent vapor-liquid mixtures, their deficiencies for general multicomponent mixtures have long been recognized. Even Murphree Ind. Eng. Chem., 17, 747-750 and 960-964 1925 , who formulated the widely used plate efficiencies that carry his name, pointed out clearly their deficiencies for multicomponent mixtures and when...

As seen earlier, the manual Thiele-Geddes method involves solving the equilibrium-stage equations one at a time. More powerful, flexible, and reliable computer programs are based on the application of sparse matrix methods for solving simultaneously all or at least some of the equations. For cases in which combined column feeds represent mixtures that boil within either a narrow range typical of many distillation operations or a wide range typical of absorbers and strippers and in which great...

As discussed in Sec. 4, the K value of a species is a complex function of temperature, pressure, and equilibrium vapor- and liquid-phase compositions. However, for mixtures of compounds of similar molecular structure and size, the K value depends mainly on temperature and pressure. For example, several major graphical K-value correlations are available for light-hydrocarbon systems. The easiest to use are the DePriester chartsChem. Eng. Prog. Symp. Ser. 7, 49, 1 1953 , which cover 12...

For a given drum pressure and feed composition, the bubble- and dew-point temperatures bracket the temperature range of the equilibrium flash. At the bubble-point temperature, the total vapor pressure exerted by the mixture becomes equal to the confining drum pressure, and it follows that X y, 1.0 in the bubble formed. Since y, KiXi and since the xt's still equal the feed concentrations denoted by Zi's , calculation of the bubble-point temperature involves a trial-and-error search for the...

Starting with the classical method of Kremser op. cit. , approximate group methods of increasing complexity have been developed to calculate groups of equilibrium stages for a countercurrent cascade, such as is used in simple absorbers and strippers of the type depicted in Fig. 13-7 gt and d. However, none of these group methods can adequately account for stage temperatures that are considerably higher or lower than the two entering-stream temperatures for absorption and stripping respectively...

The calculation for a point on the flash curve that is intermediate between the bubble point and the dew point is referred to as an isothermal-flash calculation because T2 is specified. Except for an ideal binary mixture, procedures for calculating an isothermal flash are iterative. A popular method is the following due to Rachford and Rice J. Pet. Technol., 4 10 , sec. 1, p. 19, and sec. 2, p. 3 October 1952 . The component mole balance Fz, Vy, Lx, , phasedistribution relation K, y, xi , and...

In Fig. 13-25, if P2 and the feed-stream conditions i.e., F, z, T1, P1 are known, then the calculation of T2, V, L, yt, and x, is referred to as an adiabatic flash. In addition to Eqs. 13-12 to 13-14 and the total mole balance, the following energy balance around both the valve and the flash drum combined must be included Taking a basis of F 1.0 mol and eliminating L with the total mole balance, Eq. 13-17 becomes f2 V T2 Hf- V HV - Hl - Hl 0 13-18 With T2 now unknown, Eq. 13-17 becomes f1 V T2...